In the field of semiconductor technology, research and development for further refinement of patterns have been progressed. In recent years, particularly, with high integration of a large scale integration circuit, refinement of circuit patterns, wiring patterns, or contact-hole patterns for wiring between layers forming a cell has been progressed, and a request for microfabrication technology has been increased.
In connection with this, even in the field of technology for photomask production to be used in the process for photolithography in microfabrication, a technique for forming fine and correct circuit patterns (mask patterns) has begun to be demanded.
Generally, reduction projection is performed when forming a pattern on a semiconductor substrate by photolithographic technique. The size of the pattern formed on the photomask is therefore approximately four times larger than the side of the pattern formed on the semiconductor substrate. However, this does not mean that the desired precision of the pattern formed on the photomask is smaller than the pattern formed on the semiconductor substrate. Rather, the precision of a pattern formed on the photomask as a master is desired to be higher more than an actual pattern obtained after exposure.
In today's photolithography technical field, the size of a circuit pattern to be drawn is considerably smaller than the wavelength of light to be used for exposure. Thus, in the case of forming a photomask pattern with a just four-times larger circuit pattern, light interference or the like, which is generated under exposure, influences on transfer of an original shape. As a result, the original shape cannot be transferred onto the resist film of a semiconductor substrate.
In some cases, therefore, a pattern formed on the photomask is made more complicated than an actual circuit pattern to reduce an effect of the above light interference or the like. The shape of such a pattern may be, for example, an actual circuit pattern subjected to optical proximity correction (OPC).
Hence, along with a decrease in size of a circuit pattern, a higher precision processing technique has been also desired in a lithographic technique for forming photomask patterns. Although lithography performance may be expressed in limiting resolution, as described above, the precision of a pattern formed on the photomask as a master is desired to be higher more than an actual pattern obtained after exposure. Thus, limiting resolution required for formation of a photomask pattern is almost equal to or higher than one required in lithography for forming a pattern on a semiconductor base.
In general, when forming a photomask pattern, a resist film is formed on the surface of the photomask blank in which a light-shielding film is mounted on a transparent substrate, and a pattern is then drawn (exposed) on the resist film by an electron beam. Subsequently, after obtaining a resist pattern after developing the exposed resist film, the light-shielding film is etched by using this resist pattern as a mask to obtain a light-shielding (film) pattern. The light-shielding (film) pattern thus obtained is served as a photomask pattern.
In this case, the above resist film should be thinned depending on the degree of fineness of the light-shielding pattern. This is because, when forming a fine light-shielding pattern while keeping the thickness of the resist film, the ratio (aspect ratio) of the thickness of the resist film to the size of the light-shielding pattern becomes large and causes troubles of failed pattern transfer, falling down or peeling off of the resist pattern, or the like due to deterioration of the shape of the resist pattern.
On the other hand, in the conventional photomask blank, when patterning is performed by making the resist film thinner, damage received by the resist in etching process may cause deterioration or regression of the pattern shape. In this case, a resist pattern cannot be correctly transferred to a light-shielding film, and a photomask with high patterning precision cannot be made. Thus, various investigations have been performed with respect to a photomask blank having a structure that enables high-precision patterning even if a resist film is made thinner.
For example, in JP 2006-78807 A (Patent Document 1), there is disclosed a light-shielding film having a structure in which at least one layer is made of a material mainly containing silicon and a transition metal and having an atomic ratio of silicon and metal of 4 to 15:1 (silicon: metal). The layer made of a material containing silicon and a transition metal can be processed by fluorine-containing dry etching. The fluorine-containing dry etching has a low level of damage to a resist pattern. Thus, employing the above structure may lead to a light-shielding film for ArF exposure having excellent light-shielding ability and processability.
In JP 2007-241060 A (Patent Literature 2), there is disclosed a method for further enhancing processability of a light-shielding film containing silicon and a transition metal by using a hard mask as a thin film made of a chromium-containing material.
Conventionally, as an optical film such as a light-shielding film and a half-tone phase shift film, a transition-metal compound film and a silicon compound film have been used. Here, the transition-metal compound film includes a transition metal and a light element such as oxygen, nitrogen, or carbon, which can be selected as appropriate. Especially, a chromium-containing material film and a molybdenum silicon-containing material film have been widely used as an optical film.
In the case that a light-shielding film is a film made of a chromium-containing material, if a resist film for patterning this light-shielding film is made thin, it becomes difficult to fully secure the etching resistance of the light-shielding film in patterning process. In Patent Literature 2, therefore, to make fine pattern processing possible even when a resist film is made thin, a photomask blank is proposed so that a light-shielding film is a film made of a material processable with fluorine-containing dry etching.
Using a thin chromium compound film as a hard mask film is also disclosed by this Patent Literature 2 in order to perform precise patterning to the light-shielding film which comprises a transition metal silicon compound. Using a thin chromium compound film as a hard mask film is also disclosed. Although the hard mask film made of such a chromium compound is patterned by transferring a resist pattern, it is supposed that the precision of such patterning can be secured by sufficiently reducing the thickness of the hard mask film.